This invention relates generally to refrigeration or cooling apparatus for freezers of the type in which a cold space is cooled by removing heat from the interior freezer cabinet walls and more particularly relates to low cost improvements in the temperature distribution in cooled wall freezers, especially ultra-low temperature freezers. The improved temperature distribution is accomplished by inexpensively extending the interior wall surfaces that are actively cooled to areas not cooled directly by the primary cooling apparatus. Improving the temperature distribution results in more reliable and uniform cooling of the contents as well as reduced operating costs. The invention is applicable to both conventional compression Rankine cycle refrigeration systems and Stirling cycle cooler or cryocooler systems.
FIGS. 1 through 6 illustrate an ultra-low temperature (ULT) freezer that combines structures known in the prior art with the structures of the invention. As known in the prior art, a ULT freezer typically has a vacuum insulated cabinet 10 closed off by a vacuum insulated door 12. A double or triple gasket 14 that is attached to the door 12 provides sealing against heat and moisture from the surrounding environment.
Typically a freezer is cooled by the combination of a cooling apparatus that is a cooler connected to a refrigerant circuit. The cooler is a mechanical refrigeration machine that removes heat from and condenses a refrigerant. The cooler is connected to a refrigerant circuit that has a refrigerant conduit containing a refrigerant that transports heat from in or around the interior cooled space to the cooler. The term “conduit” is used in this description to refer to a refrigerant conduit that is part of the refrigerant circuit that conveys refrigerant through its internal passage. The conduit in a refrigerant circuit is usually principally a metal tube because of the high pressure of the refrigerant. However the refrigerant conduit can include other refrigerant passages including passages formed in the cooler, as well as in fittings, manifolds or through a metal plate, such as the passages in a metal sheet that surrounds the freezer compartment of a conventional domestic refrigerator. Evaporative refrigeration equipment have a refrigerant conduit which includes both an evaporation segment in which the refrigerant accepts heat by evaporating and a condensation segment in which the refrigerant rejects heat by being cooled and condensed.
The cooler 22 that is used with the present invention is mounted in a top compartment 16 of the cabinet 10 but some types of coolers can be located at the bottom of the freezer. The present invention operates in association with a cooler 22 that is known in the prior art and therefore is illustrated symbolically. For example, the cooler 22 can be a Stirling cycle cooler or cryocooler, which is preferred, or a conventional compression Rankine cycle refrigeration system using a compressor and heat exchanger/condenser.
The invention is used in combination with a primary refrigerant circuit of a type known in the prior art. The primary refrigerant circuit has a continuous refrigerant conduit 18 which is integrated into or thermally attached to the interior vertical side walls 20 of the freezer cabinet 10 for directly cooling those walls 20. Since the interior walls 20 are exposed to the inside air of the freezer and intercept the heat from outside the freezer, the interior space adjacent the walls 20 will take on the temperature of the walls 20. The opposite ends of the refrigerant conduit 18 are connected to a cooler 22 that is diagrammatically shown in FIGS. 2-5.
For reasons that will become apparent, the cooling apparatus that is described above and known in the prior art will subsequently be referred to as the primary cooling apparatus and its principal components as the primary cooler 22 and the primary refrigerant conduit 18.
Although prior art freezers of the type described have operated successfully, they have a problem that would be desirable to eliminate. Practical considerations in the fabrication of a refrigerant conduit that is thermally attached to cabinet interior walls limit the area of the interior walls that are actively cooled by the primary cooling apparatus. Often the top cabinet wall 24 and the bottom cabinet wall (not visible) of the interior space as well as the inner wall of the door 12 are not cooled because primary refrigerant conduit is not run across and in thermal contact with the top cabinet wall 24, the bottom cabinet wall or the inner wall of the door 12. The reason is the difficulty of bending the tubular conduit into the necessary configuration. Ordinarily the entire primary refrigeration conduit is bent and shaped prior to its attachment to the outer surfaces of the interior cabinet walls. The primary refrigerant conduit 18 requires a continuous slope downward from its top to avoid low spots or traps which can cause vapor lock. Such a trap is a conduit segment that is slightly lower than its surrounding opposite ends which can allow liquid refrigerant to accumulate in the trap. The accumulated liquid prevents the vapor phase from moving through the trap which can destroy the performance of the primary cooling apparatus. Of course it would be technically possible to bend a tubular primary refrigerant conduit around a corner between a side wall and the top or bottom wall in order to extend the refrigerant conduit over the top or bottom walls. It would also be technically possible to form such a primary refrigerant conduit with the required slope to avoid low spots or traps. But such a fabrication process would add greatly to the cost because of the difficulty of bending the tubular conduits in a way that does not form flow restrictions, low spots or traps.
One consequence of having some of the cabinet wall area not actively cooled by the primary cooling apparatus is poor temperature distribution within the cold space. The poor temperature distribution results in temperature stratification within the cooled air in the freezer because there is typically no forced convection in freezers in which the interior walls are cooled by the cooling apparatus. The heat that enters the cooled interior cabinet space through these uncooled surfaces must be removed by the actively cooled walls. This causes temperature gradients and stratification within the freezer resulting in warmer areas that may compromise a specimen or product that is stored within the freezer. The warmer region within the freezer cabinet is typically near the top because of the cumulative effect of convection in the cooled interior cabinet space and the absence of active cooling of the interior, top cabinet wall. However, the cumulative effect of an interior cabinet space that is so densely packed that convection is retarded combined with an absence of active cooling of the interior bottom cabinet wall can result in a warmer region near the interior bottom. An ideal freezer would be one that has no temperature gradients or stratification within the interior space so that a desired interior temperature displayed by instrumentation would accurately represent the temperature of the entire contents of the freezer.
Another problem also exists as a consequence of spatial variations of the temperature in the cooled space within the freezer cabinet. The cooling apparatus must cool to at least the lowest temperature within the cooled space. If an operator of a freezer recognizes the existence of the undesirable temperature distribution described above and attempts to compensate for that problem by reducing the set point temperature of the freezer's control system, the energy consumed by operation of the freezer and its cost would be increased. If an invention can reduce the spatial temperature distribution in the freezer, the cost of operating the freezer would be reduced. The cost would be reduced not only because there would be less or no need to compensate for the problematic spatial temperature distribution but also because the lowest temperature within the freezer would be raised and the highest temperature would be lowered. The rise in the lowest temperature would mean that the primary cooling apparatus would require less energy for operating.
It is therefore an object and purpose of the invention to simplify construction of a freezer in a manner that reduces the cost of fabricating a cooled wall freezer by extending active cooling to the top and/or bottom interior walls without requiring the primary refrigerant conduit to be bent in a configuration for attachment to both the side walls and also the top and/or bottom walls of the freezer's interior cabinet walls.
It is a further object and purpose of the invention to reduce the energy cost for operating a freezer by substantially reducing or eliminating spatial variations of the temperature distribution within the freezer.